Effects of high-intensity focused ultrasound on the intervertebral disc: a potential therapy for disc herniations.

Purpose: To determine the potential application of high-intensity focused ultrasound for the minimally invasive treatment of herniated intervertebral discs by developing a probe that produces sufficiently high temperature locally to shrink collagen fibers (65-75 degrees Celsius).

Materials And Methods: A 5-mm ultrasound probe was produced with a geometric focal length of 15 mm. The probe produced 2.5 W of acoustic power and was operated at a frequency of 4.1 MHz. Measurements of temperature increase were performed in discs from bovine tails. In vivo experiments were performed to assess histologic changes in the disc as well as in nerve root and muscle.

Results: Sufficient temperature increase to produce collagen shrinkage was observed close to the focus of the ultrasound. Temperature measurements in vertebral end plates showed a temperature increase of only 4 degrees Celsius after 60-second exposure of the disc. In vivo experiments revealed histologic changes in the disc consistent with collagen shrinkage, with no adverse effects seen in surrounding tissues.

Conclusions: The experiments demonstrated the feasibility of high-intensity focused ultrasound in the treatment of contained herniated discs. This technique has several advantages over other thermal treatment modalities.